Fix crash when sending embed

Created by: uwx

Summary

Fixes the crash when receiving a message with an embed that contains a custom color.

Details

The crash was due to the following:

1. Support for serializing Optional, but no support for deserializing it.
2. Optional serializer would assume the member being serialized is a property which it maybe not.

Changes proposed

• Add support for Deserializing Optional, and serializing Optional fields
• Cache JsonSerializerSettings
• Fix guild edit payload sending incorrect data (likely due to a bad merge)

Notes

TODO: Are the member edit payloads correct? Should double check at some point.